Chromatography detector

A chromatography detector is a device used in gas chromatography (GC) or liquid chromatography (LC) to visualize components of the mixture being eluted off the chromatography column.

There are as many as five ways to classify detectors: concentration vs. mass flow rate; selective vs. universal; destructive vs. non-destructive; bulk property vs. specific property; and analog vs. digital.^[1]^[2] Of the five classification systems listed, the first three are the most important.^[1]

Concentration vs. mass flow rate

This classification differentiates between detectors that measure the concentration of analyte in the carrier gas versus those that take a direct measurement of the absolute amount of analyte regardless of the volume of the carrier gas.^[1]^[2]

Selective vs. universal

This classification differentiates between detectors based on the percentage of analyte capable of being detected by the system. Selective systems will only respond to a particular types or classes of compounds, while a universal system will generally be able to detect all solutes.^[1]^[2]

Destructive vs. non-destructive

A destructive detector performs continuous transformation of the column effluent (by burning, evaporating, or mixing with reagents), with a subsequent measurement of some physical property of the resulting material (plasma, aerosol, or reaction mixture). A non-destructive detector directly measures some property of the column effluent (UV absorption, for example) and thus affords for further analyte recovery.

Examples of destructive detectors include:

Flame ionization detector: The column effluent is injected into hydrogen flame and the flame conductivity is measured. This detector is only used in gas chromatography.
Flame photometric detector: Column eluate is burned in a hydrogen flame. The light of the flame is filtered through an optical filter, allowing the detection of sulfur and phosphorus.
Atomic-emission detector: The column effluent is injected into plasma and the plasma spectrum is recorded. This detector allows for immediate determination of the elements present in the analyte.^[3]
Mass spectrometry detector: The column effluent is continuously injected into a mass spectrometer. This detector allows for immediate determination of the molecular weight of analyte and/or the fragmentation pattern, thus providing a significant amount of information about analyte.
Nitrogen phosphorus detector: This detector is only used only in gas chromatography.
Evaporative light scattering detector: The column effluent is continuously evaporated and the light scattering of the resulting aerosol is measured. This detector is only used in liquid chromatography.^[4]

Examples of non-destructive detectors include:

Ultraviolet detector: This detector works on either a fixed or variable wavelength. The UV absorption of the effluent is continuously measured at single or multiple wavelengths. A diode array detector (DAD or PDA) is an example of an ultraviolet detector. Ultraviolet detectors are typically the most popular detectors for liquid chromatography.^[5]
Thermal conductivity detector: This detector measures the thermal conductivity of the effluent and is only used in gas chromatography.
Fluorescence detector: Using the principle of fluorescence spectroscopy, this detector irradiates effluent with a light of set wavelength and measures the fluorescence of the effluent at a single or multiple wavelength. It's only used in liquid chromatography.
Electron capture detector: This is the most sensitive detector known. Allows for the detection of organic molecules containing halogen, nitro groups, etc.
Conductivity monitor: This detector continuously measures the conductivity of the effluent. Used only in liquid chromatography when conductive eluates (water or alcohols) are used.^[6]
Refractive index detector: This detector continuously measures the refractive index of the effluent, and it's used only in liquid chromatography. The lowest sensitivity of all detectors. Useful when nothing else works and at high analyte concentrations.
Radio flow detector: This detector measures radioactivity of the effluent. This detector can be destructive if a scintillation cocktail is continuously added to the effluent.
Chiral detector: This detector continuously measures the optical angle of rotation of the eluent. Used only in liquid chromatography when chiral compounds are being analyzed.